Compact fluid system for shifting a transmission

ABSTRACT

Means for applying fluid pressure to selected pairs of the drive conditioning devices of a vehicle transmission in response to control lever movement includes a pressure modulating valve for establishing a gradual pressure rise at the selected drive devices and a safety differential spool valve provides for a delayed lesser pressure rise at the one of the devices which reestablishes drive through the transmission. The safety differential spool valve performs the further function of blocking fluid pressure from one of the selected devices if the vehicle engine should be started up with the control levers at a position other than neutral. Two valves thus provide transmission control functions which have heretofore required more complex valving assemblies.

United States Patent [1 1 Starling [54] COMPACT FLUID SYSTEM FORSHIFTING A TRANSMISSION [75] inventor: James G. Starling, Pekin, Ill.

[7 3] Assignee: Caterpillar Tractor Co., Peoria, Ill. 22 Filed: March a,1911 [2]] Appl. No.: 121,810

['52] US. Cl. ..74/753, 192/109 F 511 im. Cl ..F16h 3/44 581 Field ofSearch ..74/733, 753; 192/109 F [56] References Cited 7 Y UNITED STATESPATENTS 3,468,194 I 9/1969 Horsch et al ..74/753 I 3,181,394 5/1965Ramjsel et al. ..74/753 6/1968 Golan et al ..l92/l09 F l RIATI [in3,709,065 51 Jan.9, 1973 Primary ExaminerC. J. Husar Attorney-Fryer,Tjensvold, Feix, Phillips & Lempio 57 v ABSTRACT Means for applyingfluid pressure to selected pairs of 4 the drive' conditioning devices ofa vehicle transmission in response to control lever movement includes apressure modulating valve for establishing a gradual pressure rise atthe selected drive devices and a safety differential spool valveprovides for a delayed lesser pressure rise at the one of the deviceswhich reestablishes drive through the transmission. The safetydifferential spool valve performs the further function of blocking fluidpressure from one of the selected devices if the vehicle engine shouldbe started .up with the control levers at a position other than neutral.

Two valves thus provide transmission control functions which haveheretofore required more complex valving assemblies.

6 Claims, 4 Drawing Figures PATENTED JAN 9 I975 SHEET 1 OF 3 fi I L RTIOT A 2ND -IZ v j 14 [2C RATIO LJ I E J E INVENTOR IO JAMES G.STARLINGPATENTEDJAN ems 3.709.065

sum 3 OF 3 III INVENTOR JAMES (3. STAR LING ATTORNEYS COMPACT FLUIDSYSTEM FOR SI-IIFTING A TRANSMISSION BACKGROUND OF THE INVENTION Thisinvention relates to vehicle transmissions of the form which are shiftedby applying fluid pressure to selected ones of a plurality of driveconditioning devices within the transmission and more particularly tofluid circuits which establish and control the fluid pressures inresponse to movement of the operator's control levers or the like.

Manytransmissions for tractors and other vehicles include a number ofclutches or brakes which are actuated by fluid pressure to establishselected drive conditions within the transmission in response tomovement of operators shift levers. One of a first group of the driveconditioning devices is actuated to select a drive ratio through thetransmission and one of a second group of the devices is actuated toestablish either forward or reverse drive.

To avoid shock loads and excessive wear during shifting, careful controlof the rate'of pressure rise at the selected devices is desirable and itis desirable that the selected one of one particular group be engagedslightly after the selected device of the other group.

The last engaging group of devices are of a more massive and wearresistant construction as the last engaging device must recouple thevehicle engine and drive line and is therefore subjected to the mostsevere loads.

tuation of one of the selected drive conditioning devices of thetransmission when fluid pressure is first supplied to the system unlessthe'shift control lever is in neutral or until such time as the controllever is momentarily returned to neutral.

A variety of shift control circuits have been devised for the foregoingpurposes. These prior systems have in general been undesirably complexand require considerable maintenance. U.S. Pat. No; 3,091,976 disclosesa typical prior system wherein the safety function necessitates acomplex spool valve assembly additional to two spool valveassemblies-required for modulating pressures at the drive conditioningdevices during a shift. Further, the safety spool valve mechanismrequires springs sufficiently heavy to move the operators shift leverfor the purpose of establishing .a neutral condition. Other controlsystems for this general purpose tend to be still more complex andcostly and are often prone to severe maintenance problems.

SUMMARY OF THE INVENTION This invention provides a fluid circuit of thegeneral type discussed above which is extremely compact, simple andreliable. The safety function of maintaining a neutral condition in thetransmission if the vehicle engine is started up with the operator'scontrol lever at a position other than neutral is performed by the samespool valve assembly that provides fluid pressure control for the lastengaging drive conditioning device.

This is accomplished without physical movement of the operator's controllever to provide the safety function and the system is fail safe in thesensethat vehicle immobility is assured if fluid passages associatedwith the safety valve spool become plugged.

Accordingly, it is an object of this invention to provide a more compactand reliable fluid circuit for controlling the application of fluidpressure to drive establishing devices in a vehicle transmission inresponse to movements of an operators shift control lever or the like.

The invention together with further objects and advantages thereof willbest be understood by reference to the following description of apreferred embodiment taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING- In the accompanying drawing:

FIG. 1 illustrates a shift control system for a vehicle transmissionwith certain elements being shown in section and certain other elementsbeing shown schematically, movable elements being shown in the positionswhich are assumed during shutdown of the vehicle,

FIG. 2 is a section view of a portion of the valve mechanism of FIG. 1showing positions which the com-. ponents assume in the course ofpreparing for a transmission shift immediately after movement of theoperators control lever,

FIG. 3 is a view of the valve mechanism of FIG. 2 showing the positionswhich-components assume at the completion of the shift'transient, and

FIG. 4- is a view illustrating a modification of a portion'of the shiftcontrol system.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawing andmore particularly to FIG. 1 thereof there is shown a transmissioncontrol system'll applicable to multispeed transmissions 10 of the formwhich have a first group of fluid pressure operated drive establishingdevices 12 for selecting a drive ratio and a second group of fluidpressure operated devices 13 for determining drive direction and whereina selected single one of the devices 12 and a selected single one of thedevices 13 are actuated to establish'drive through the transmission ata-desired ratio and direction. For example, three devices, 12A, 12B and12C, may be present for establishing first, second'and third gear ratiosand devices-13A and 13B provide for either forward drive or reversedrive respectively. The drive conditioning devices 12 and 13 mayvariously be clutches or brakes within the transmission, suitabledesigns for such devices being well knownto the art. The transmission 10may also be of a known construction and accordinglythe transmission l0and devices 12 and 13 areshown in schematic form in FIG. I. to avoidunnecessary complication of the drawing.

Fluid under pressure for actuating the drive conditioning devices 12 and13 is delivered by a pump '14 from'a reservoir 16, the pump beingcustomarily driven by the engine of the vehicle with which thetransmission is associated. Pump 14 has a branched outlet conduit' 17with a first-branch 17A for transmitting fluid to a selected one of the.ratio devices l2 through a drive ratio selector valve 18 and has asecond branchl'lB for transmitting fluid to a selected one of thedirectional devices 13 through a pressure control valve group 19,cut-off valve 21 and directional selector valve 22.

Drive ratio selector valve 18 may be a spool valve for selectivelycommunicating conduit 17A with one of the devices 12 according to theposition of a spool 23 and may be shifted manually by an operator'scontrol lever mechanism 24. Valve 18 has four positions in this exampleincluding a neutral position at which conduit 17A is communicated withthe second drive ratio device 128, a first gear position at which fluidfrom conduit 17A is directed to device 12A, a second gear position atwhich conduit 17A is again communicated with device- 12B and a thirdgear position at which conduit 17A is communicated with device 12C,those devices 12 which are not communicated with conduits 17A at anyselected setting other than neutral being communicated with drains 26 or27. Similarly, directional selector valve 22 includes a spool 28 whichmay be manually shifted between forward and reverse positions byoperators control lever 24. At the forward position, directionalselector valve 22 directs fluid, obtained throughconduit 17B, pressurecontrol group 19 and cut-off valve 21, to device 13A while ventingdevice 138. At the reverse setting selector valve 22 directs fluid todevice 138 while venting device 13A.

Accordingly, any time that a shift is initiated by manipulation ofcontrol lever 24, while the vehicle is in operation, at least one of thedrive conditioning devices 12 and 13 is disconnected from pump outputconduit 17 and vented while another one of the drive conditioningdevices is communicated with the conduit 17. As filling of the newlyselected device 12 or 13 cannot occur instantaneously, the pressure inconduit 17 momentarily drops. The pressure control group 19 thenfunctions to modulate the rate at which the pressure rises in theselected devices 12 and 13 and further functions to delay the pressurerise in the selected directional device 13 relative to the ratio device12 and to establish a lesser final pressure in the directional device.For this purpose the pressure control group 19 includes a housing 31having a pair of parallel bores 32 and 33 containing spool valveassemblies 34 and 36 respectively. Spool valve assembly 34 controls therate of pressure rise in the selected devices 12 and 13 by functioningas a modulating relief valve while spool valve assembly 36 functions asa differential valve to delay the pressure rise at the directionaldevice 13 relative to that at the ratio device 12, and to provide asmaller final pressure at the device 13. In accordance with the presentinvention, spool valve assembly 36 further provides a safety function bypreventing application of fluid pressure to the selected directionaldevice 13 if the shift control lever 24 is not at the neutral positionwhen the pump 14 is first started up.

Considering now the configuration of the modulating relief spool valveassembly 34 in more detail, the pump outlet conduit 17B is communicatedwith an inlet passage 37 to safety-differential valve assembly 36through a groove 38 in the wall defining bore.32. A spool 39 is disposedin bore 32 and has an edge 41 which defines a variable flow passage fromgroove 38 to a relief outlet groove 42 which in this particular exampleis communicated with a torque converter 43 of the transmission. In otherinstances, relief groove 42 may simply communicate with a drain. Acompression spring 44 in bore 32 acts between the spool 39 and a loadpiston 46 in bore 32 .to urge the spool in a direction at which land 41tends to close the flow passage between grooves 38 and 42.

The fluid pressure within groove 38 is transmitted to a chamber 47 inthe end of spool 39 opposite the spring 44 through a passage 48 in thespool and a check valve 49 disposed therein, the check valve beingby-passed by a smaller reset orifice passage 51. Thus, spring 44 tendsto close the relief flow passage past land 41 while fluid pressure inchamber 47 reacts against a slug 50 and against the spool in adirection-tending to open the relief flow passage. Spool valve assembly34 functions as a relief valve in that the flow passage past land 41 ispression of spring 44, the load piston movement beingbrought about bymeans to be hereinafterdescribed.

Considering now the construction of the safety-differential spool valveassembly 36 in more detail, inlet passage 37 communicates with a groove55 of bore 33 which contains a spool 45 acted upon by a spring 60.

Spring 60 urges the spool 45 in a direction at which the spool blockscommunication between groove 55 and an adjacent groove 52 which iscommunicated with a conduit 53 for supplying pressurized fluid to thedirectional devices 13 through cut-off valve 21 and directional selectorvalve 22. v

To shift the spool 45 against spring 60 to supply fluid to conduit 53,fluidpressure from inlet passage 37 is transmitted to a chamber 54 inthe end of spool 45 oppositefrom spring 60 through an orifice 56 whichcommunicates with an axial passage 57 in the spool having a check valve58 therein. At start-up of the pump 14, the fluid pressure in chamber 54shifts the spool 45 against spring 60 provided that control lever 24 isin neutral or is ,subsequently returned to neutral as will hereinafter.be discussed in more detail. The movement is accelerated when an edge 59of the spool 45 reaches groove 55 to open a larger passage 61 to chamber54 through check I valve 58. Prior to the above described initialmovement of spool 45 against spring 60, groove 52 'is communicated withan adjacent groove 62 which is open to a drain 63 thereby relieving anyfluid pressure in conduit 53.' Spool 45 has radial passages'65 whichmaintain the communication between grooves 52 and 62 after spool 45intersects groove 52, as shown in FIG. 2,'until the" spool also closesgroove 62 as shown in FIG. 3. After the spool has moved sufficiently toclose drain groove 62, edge 59 of the spool provides a flow passage frominlet groove 55 to groove 52 thereby supplying fluid pressure to conduit53 for actuating the selected directional device 13. Owing to the actionof spring 60 on spool 45 a differential valve action is produced wherebythe pressure in conduit 53 is maintained at a lesser value than thepressure in conduit 17A. As a consequence, valve assembly 36 does notopen to pressurize a directional device 13 until after the pressure atthe ratio device 12 has risen a predetermined amount thereby delayingthe engagement of the device 13 relative to device 12.

Referring again to FIG. 1, to provide for the modulating action wherebythe pressures at the selected devices 12 and 13 both rise in a gradualmanner following manipulation of control lever 24, an apertured checkvalve 64 is disposed in the opposite end of bore 33 from spool 45. Checkvalve 64 may shift in the direction of spool 45 to seat against anannular spring retainer 66 at which position the check valvecommunicates a drain 67 with a passage 68. Passage 68 communicates witha groove 69 in the end of modulating relief valve passage 32 to relieveany fluid pressure on load piston 46 of the modulating relief valve whenspool 45 shifts in response to the initial pressure drop at thebeginning of a shift. During the shift period when spool 45 of thesafety differential valve 36 moves against the spring 60 to admit fluidpressure to groove 52 while closing drain groove 62, fluid pressure isalso caused to act against the check valve 64 through the radialpassages 65 in spool 45 which communicate groove 52 with the check valveend of bore 33. At this point the fluid pressure shifts check valve 64away from. retainer 66 to isolate passage 68 from drain 67. The checkvalve has a restricted flow passage. 72 which gradually transmits thefluid pressure to passage 68 and thus to groove 69 where the pressureacts against load piston 46 to gradually increase the compression ofspring 44 against modulating relief valve spool 39. Thus, the modulatingrelief valve assembly 34 is caused to produce a gradual rise in pressureat the selected drive devices 12 and 13 to engage the associatedtransmission without abrupt shock loads with the pressure rise at thedirectional device 13 being delayed in time relative to that at theratio device 12. The rate of the pressure rise is thus basicallydetermined by the sizing of restricted passage 72 in check valve 64 andthe final and maximum pressure applied to the devices 12 and 13 is fixedby the abutment of a flange 46 of load piston 46 against a step 32' ofbore 32.

The cut-off valve 21 has a spool 73 in a bore 74 of a housing 76 whichis shifted axially by movement of the operators control lever 24. At theneutral position of control lever 24, spool 73 blocks communicationbetween conduit 53 and the inlet passage 77 of directional selectorvalve 22 while venting the passage 77. At any of the other positions ofoperator shift lever 24, conduit 53 is communicated with passage 77.Thus, neither of the directional devices 13 can be pressurized by fluidfrom conduit 53 at the neutral position and no torque can be transmittedthrough the transmission under that condition.

Considering now the means by which the above described mechanismsprovidethe safety function at start-up in addition to providing the primarypressure control functions as described above, an additional conduit 78extends between the pressure control group 19 and cut-off valve 21.Conduit 78 communicates a groove 79 of bore 33 of housing 31 with agroove 81 of bore 74 of cut-off valve 21. Groove 81 is positioned to beclosed by a land 82 of spool 73 when the operators control lever 24 isin the neutral position and is vented at any other position of the shiftlever 24 and spool 73.

Groove 79 at the safety-differential spool valve as sembly bore 33 ispositioned to communicate with chamber 54 of the spool 45 through radialpassages 83 in the spool when the spool is shifted to the initialposition at the end of bore 33 by spring 60. The spool shifts in thismanner following a prolonged absence of fluid pressure in passage 37 asfluid pressure in chamber 54 is gradually relieved through an orifice 86which is sized to prevent a significant pressure loss in chamber 54during the brief pressure drop at the beginning of a shift.

When the vehicle has been shut down and pump 14 is inactive, thecomponents of the pressure control group 19 assume the positions shownin FIG. 1 at which both spool valve assemblies 34 and 36 are moved fullyto the left as viewed in the drawing.

Upon starting up of the pump 14 with the shift lever 24 in neutral, thespools 39 and 45 of pressure control group 19 shift initially to thepositions shown in FIG. 2 as the second gear drive ratio device 12Bfills with fluid. Thereafter spool valve assemblies 34 and 36 movefurther to the positions shown in FIG. 3. Referring again to FIG. 1, notorque transmission through the transmission can occur at this timeinasmuch'as the position of spool 73 of cut-off valve 21 blocks fluidpressure from both directional devices 13. If shift control lever 24 isthen moved into a selected drive ratio setting, device 12B is drainedand filling of the selected one of the devices commences. The initialfill period for the selected one of the ratio devices 12 produces atemporary pressure drop in conduit 17 and the spool assemblies 34 and 36reset to the positions shown in FIG. 2. Spool assembly 36 does notreturn all the way.

to the FIG. 1 position as the pressure drop is not of sufficientduration to relieve all fluid pressure in chamber 54. Pressure group 19then beginsa modulated reapplication of pressure to the appropriatedevices 12 and 13 as hereinbefore described until the valve positions ofFIG. 3 are again reached at which point one each of the devices 12 and13 is fully pressurized and the transmission transmits torque.

If the lever 24 is not at the neutral setting when pump 14 is firstactivated, safety-differential valve spool 45 is unable to shift againstthe action of spring 60 and thus conduit 53 remains vented to drain 63and no fluid pressure can reach either of the directional devices 13.Valve spool 45 is unable to move against spring 60 under this conditionas fluid pressure cannot build up in chamber 54 to any extent since thechamber is vented through conduit 78 and cut-off valve 21. Fluidpressure can begin to build up in chamber 54 only after the controllever is returned to the neutral position at least momentarily therebyblocking conduit 78.

The above described mechanism provides considerable fail safety relativeto prior devices in that a blockage of certain of the restricted flowpassages and orifices similarly prevents movement of the spool valveassembly 36 away from the safety position.

It will be apparent that modifications of the above described structureare possible'within the scope of the invention. FIG. 4, for example,depicts a modification of the upper spool valve assembly 34. Themodified valve assembly 34' controls the rate of pressure rise followingoperation of the shift controls by functioning as a modulating reducingvalve rather than as a modulating relief valve as in the correspondingassembly of the first described'embodiment. The construction of thetransmission control system as a whole may be similar to the embodimentpreviously described except as shown in FIG. 4 and hereinafter describedwith reference thereto.

The housing 31' of the pressure control group 19 again has a bore 32containing the modified spool valve assembly 34'. The pump outletconduit 178 communicates with an annular groove 91 in the wall whichdefines bore 32", the groove 91 being spaced from the groove 38 whichcommunicates with flow passage 37' and being separated therefrom by aland 92. As valve assembly 34 provides the desired pressure modulationby metering fluid flow from pump outlet conduit 178' to passage 37rather than by variably relieving pressure in the conduit as in theprevious instance, a modification in the fluid circuit outside housing31' is required. In particular, pump outlet conduit 17B is notcommunicated directly with the speed selector valve supply conduit 17Aas in the previous instance. In the present embodiment, conduit 17A iscommunicated with passage 37 to receive fluid metered across land 92. Inaddition, pump outlet conduit 17B is connected to drain through aconventional pressure regulator valve 93 which maintains a predeterminedconstant supply pressure in the conduit. If the transmission includes atorque converter as in the previous example, fluid for the converter maybe derived from the outlet of pressure regulator valve 93.

To meter fluid across land 92 to provide for a gradually rising pressurein passage 37, a modified spool 39 is disposed in bore 32" and has anannular groove 94 defining an edge 96 which coacts with land 92 toprovide a flow passage between grooves 91 and 38' that varies accordingto the axial position of the spool in the bore.

A load piston 46 is also disposed in bore 32" and a compressionspring44' is situated between the load piston and spool 39" to exert a forceagainst the spool tending to open the flow passage between land 92 andedge 96. Radial passages 97 in spool 39' communicate groove 38 with achamber 98 in the end of the spool opposite from spring 44'.Accordingly, fluid pressure acts against spool39' in a directionopposite to the force of spring 44'. A reaction plug 99 may be disposedin chamber 98 and a constricted axial bleed passage 101 communicateschamber 98 with the region of bore 32" between spool 39' and load piston46, the region being communicated with a drain 63 as in the previousexample.

Thus when the fluid pressure in passage 37' and groove 38' ismomentarily vented at the beginning ofa shift as previously described,spring 44 shifts spool 39 to provide the maximum flow passage betweensupply conduit 17B, past edge 96 to passages 37' and 17A. After theselected transmission devices supplied through passages 37' and 17A havefilled, the pressure begins to rise in groove 38' and this pressure riseis transmitted to chamber 98 through passages 97. Accordingly, fluidpressure acts to move spool 39' to reduce the flow passage past edge 96to avoid an overly abrupt pressure rise in passage 37. In the absence offurther provisions, this action would close off the flow from supplyconduit 178' to passage 37 at some point in the pressure rise determinedby the dimensions of the spool 39' and the force of spring 44' However,this action does not occur inasmuch as load piston 46" is graduallyshifted, to increase the force of spring 44' against spool 39 by therising fluid pressure in a chamber 46" at the end of bore 32" whichpressure is received through a passage 68 in the-manner previouslydescribed for the first embodiment of the invention.

Thus the rising fluid pressure tending to close the flow passage pastedge 96 is temporarily resisted by an increasing spring force on spool39' and the desired modulated pressure increase in passage 37 isrealized, When the flange 46 on load piston 46' abuts the step 32' ofbore 32", the modulating action ceases, The flow passage past edge 96remains slightly open to compensate for flow through bleed passage 101and any other leakage in the system while maintaining a constantpredetermined pressure in the passage 37' and passage 17A. Thus the endresult of the modified valve assembly 34 is similar to that of thecorresponding modulating relief valve assembly of the first describedembodiment.

What is claimed is:

1. In a fluid circuit for shifting a transmission having a plurality offluid pressure actuated drive'conditioning devices and having anoperators shift control which has aneutral position and at least onedrive position, the combination comprising means for supplying fluidunder pressure, selector valve means for directing said fluid from saidsupply means to predetermined ones of said drive conditioning devices inresponse to movement of said shift control, pressure control means forproducing a gradual rise of fluid pressure at said predetermined ones ofsaid drive conditioning devices following said movement of saidoperators shift control, said pressure control means including a movabledifferential valve element defining a portion of the flow passage fromsaid supply means to at least one of said predetermined ones of saiddrive conditioning devices and having resilient means acting on saidvalve element in a direction tending to close said flow passage and toshift said valve element to an initial position past that at which saidflow passage is closed and having means defining a chamber wherein saidfluid pressure acts on said valve element in a direction tending to opensaid flow passage,

means defining a drain passage which is commu-' nicated with saidchamber when said valve element is in said initial position and which isblocked from said chamber by movement of said valve element when saidvalve element moves away from said initial position, and

a cut-off valve linked to said shift control and operated thereby forblocking said drain passage when-said shift control is in said neutralposition and for opening said drain passage when said shift control isaway from said neutral position whereby an initial buildup of fluidpressure in said chamber to shift said valve element away from saidinitial position thereof can only occur when said shift control is insaid neutral position.

2. The combination defined in claim 1 wherein said chamber receives saidfluid through a constricted passage in said valve element and furthercomprising a check valve disposed in said passage of said valve element.

3. The combination defined in claim 2 wherein said valve element has anadditional passage for transmitting said fluid to said chamber throughsaid check valve, said additional passage bypassing said constrictionand being positioned to be closed when said valve is at said initialposition.

4. A fluid circuit for shifting a transmission having a first pluralityof fluid pressure actuated drive conditioning devices for selecting aspeed ratio through said transmission and having a second plurality offluid pressure actuated drive conditioning devices for selecting a drivedirection through said transmission, comprising:

an operators shift control means having a neutral position and forwardand reverse drive positions and a plurality of speed ratio positions,

a pump for supplying fluid under pressure,

a speed ratio selector valve linked to said operators shift controlmeans for directing fluid from said pump to predetermined ones of saidspeed ratio conditioning devices, as determined by the position of saidshift control,

a drive direction selector valve for directing fluid from said pump to apredetermined one of said second plurality of drive conditioning devicesas determined by the position of said shift control,

pressure control valve housing means defining a portion of the fluidflow path from said pump to said drive direction selector valve andhaving first and second bores therein wherein said fluid from said pumpis transmitted to a portion of said first bore,

a pressure modulating valve assembly disposed in said first bore andhaving a spool with an edge defining a variable flow passagecommunicated with said first portion of said first bore for controllingthe pressure of fluid supplied to a region of said second bore, saidspool being spring biased in a direction tending to increase fluidpressure at said region of said second bore and having load piston meansfor varying said spring biasing in accordance with fluid pressuresupplied to a second portion of said first bore,

a safety differential valve disposed in said second bore of said housingand having a spool movable in a first direction to block said region ofsaid second bore from said drive direction selector valve and beingfurther movable in the same direction to an initial position and beingmovable in an opposite direction to communicate said region of saidsecond bore with said drive direction selector valve and havingresilient biasing means acting thereon in said first direction,

means defining a fluid chamber wherein fluid pressure acts on said spoolof said safety differential valve in said opposite direction tending toopen communication between said region of said second bore and saiddrive direction selector valve, said chamber being communicated withsaid region of said second bore whereby fluid pressure is transmitted tosaid chamber, means defining a dram passage which 15 communicated withsaid chamber when said safety-differential valve spool is at saidinitial position and which is blocked from said chamber when saidsafety-differential valve spool moves away from said initial position,and

a cut-off valve responsive to movement of said shift control to closesaid drain passage when said shift control is in said neutral positionwhereby said fluid pressure in said chamber is unable to shift saidsafety-differential valve spool away from said initial position exceptwhen said shift control is in said neutral position.

5. The combination defined in claim 4 wherein said cut-off valve blockssaid flow path to said drive direction selector valve when said shiftcontrol is in said neutral position thereof.

6. The combination defined in claim 4 wherein said spool of saidsafety-differential valve opens said second portion of said first boreto a drain when said safety differential valve spool is at said closedposition and transmits fluid pressure to said second portion of saidfirst bore through a flow restriction when said region ofsaid secondbore is communicated with said drive direction selector valve, wherebysaid load piston is gradually shifted to increase said spring biasing onsaid spool of said modulating valve assembly whereby a gradual pressurerise is produced at said predetermined ones of said drive conditioningdevices.

1. In a fluid circuit for shifting a transmission having a plurality offluid pressure actuated drive conditioning devices and having anoperator''s shift control which has a neutral position and at least onedrive position, the combination comprising means for supplying fluidunder pressure, selector valve means for directing said fluid from saidsupply means to predetermined ones of said drive conditioning devices inresponse to movement of said shift control, pressure control means forproducing a gradual rise of fluid pressure at said predetermined ones ofsaid drive conditioning devices following said movement of saidoperator''s shift control, said pressure control means including amovable differential valve element defining a portion of the flowpassage from said supply means to at least one of said predeterminedones of said drive conditioning devices and having resilient meansacting on said valve element in a direction tending to close said flowpassage and to shift said valve element to an initial position past thatat which said flow passage is closed and having means defining a chamberwherein said fluid pressure acts on said valve element in a directiontending to open said flow passage, means defining a drain passage whichis communicated with said chamber when said valve element is in saidinitial position and which is blocked from said chamber by movement ofsaid valve element when said valve element moves away from said initialposition, and a cut-off valve linked to said shift control and operatedthereby for blocking said drain passage when said shift control is insaid neutral position and for opening saId drain passage when said shiftcontrol is away from said neutral position whereby an initial buildup offluid pressure in said chamber to shift said valve element away fromsaid initial position thereof can only occur when said shift control isin said neutral position.
 2. The combination defined in claim 1 whereinsaid chamber receives said fluid through a constricted passage in saidvalve element and further comprising a check valve disposed in saidpassage of said valve element.
 3. The combination defined in claim 2wherein said valve element has an additional passage for transmittingsaid fluid to said chamber through said check valve, said additionalpassage bypassing said constriction and being positioned to be closedwhen said valve is at said initial position.
 4. A fluid circuit forshifting a transmission having a first plurality of fluid pressureactuated drive conditioning devices for selecting a speed ratio throughsaid transmission and having a second plurality of fluid pressureactuated drive conditioning devices for selecting a drive directionthrough said transmission, comprising: an operator''s shift controlmeans having a neutral position and forward and reverse drive positionsand a plurality of speed ratio positions, a pump for supplying fluidunder pressure, a speed ratio selector valve linked to said operator''sshift control means for directing fluid from said pump to predeterminedones of said speed ratio conditioning devices, as determined by theposition of said shift control, a drive direction selector valve fordirecting fluid from said pump to a predetermined one of said secondplurality of drive conditioning devices as determined by the position ofsaid shift control, pressure control valve housing means defining aportion of the fluid flow path from said pump to said drive directionselector valve and having first and second bores therein wherein saidfluid from said pump is transmitted to a portion of said first bore, apressure modulating valve assembly disposed in said first bore andhaving a spool with an edge defining a variable flow passagecommunicated with said first portion of said first bore for controllingthe pressure of fluid supplied to a region of said second bore, saidspool being spring biased in a direction tending to increase fluidpressure at said region of said second bore and having load piston meansfor varying said spring biasing in accordance with fluid pressuresupplied to a second portion of said first bore, a safety differentialvalve disposed in said second bore of said housing and having a spoolmovable in a first direction to block said region of said second borefrom said drive direction selector valve and being further movable inthe same direction to an initial position and being movable in anopposite direction to communicate said region of said second bore withsaid drive direction selector valve and having resilient biasing meansacting thereon in said first direction, means defining a fluid chamberwherein fluid pressure acts on said spool of said safety differentialvalve in said opposite direction tending to open communication betweensaid region of said second bore and said drive direction selector valve,said chamber being communicated with said region of said second borewhereby fluid pressure is transmitted to said chamber, means defining adrain passage which is communicated with said chamber when saidsafety-differential valve spool is at said initial position and which isblocked from said chamber when said safety-differential valve spoolmoves away from said initial position, and a cut-off valve responsive tomovement of said shift control to close said drain passage when saidshift control is in said neutral position whereby said fluid pressure insaid chamber is unable to shift said safety-differential valve spoolaway from said initial position except when said shift control is insaid neutral position.
 5. The combination defined in claim 4 whereinsaid cut-off valve blocks said flow path to said drive directionselector valve when said shift control is in said neutral positionthereof.
 6. The combination defined in claim 4 wherein said spool ofsaid safety-differential valve opens said second portion of said firstbore to a drain when said safety differential valve spool is at saidclosed position and transmits fluid pressure to said second portion ofsaid first bore through a flow restriction when said region of saidsecond bore is communicated with said drive direction selector valve,whereby said load piston is gradually shifted to increase said springbiasing on said spool of said modulating valve assembly whereby agradual pressure rise is produced at said predetermined ones of saiddrive conditioning devices.